CA2261895A1 - Method of making a laminate comprising a conductive polymer composition - Google Patents

Abstract

A method of making a laminate from a conductive polymer composition in which a particulate filler is dispersed in a polymeric component. The method comprises the following steps conducted sequentially in a single continuous procedure:
(A) loading the polymeric component and the conductive filler into a mixing apparatus; (B) mixing the polymeric component and the conductive filler to form a molten mixture; (C) transporting the mixture from the mixing apparatus through a die; (D) forming a polymeric sheet; and (E) attaching metal foil to a least one side of the sheet to form a laminate. The laminate can be used to prepare circuit protection devices or heaters.

Description

CA 0226189~ 1999-01-29 W 098/05503 PCTrUS97/13419 METHOD OF MAKING A LAMINATE COMPRISING
A CONDUCTIVE POLYMER COMPOSITION

BACKGROUND OF THE INVENTION

Field of the Invention This invention relates to a method of making a lAmin~te comprising a conductive 10 polymer composition and electrical devices comprising such l~min~te.

Introduction to the Invention Conductive polymer compositions which exhibit PTC (positive te~ dl~lre 15 coefficient of resistance) behavior are well-known for use in electrical devices such as circuit protection devices. Such compositions comprise a polymeric component, and dispersed therein, a particulate conductive filler such as carbon black or metal. The amount and type of filler in the composition are determined by the required resistivity for each application, as well as by the nature of the polymeric component. Compositions suitable for use in circuit 20 protection devices have low resistivities at room telllp~.dl~lre, e.g. less than 100 ohm-cm, and generally comprise relatively high levels of conductive filler. When such highly filled compositions are prepared by conventional methods such as melt-mixing, they are subject to substantial shear. Such shear generates heat, which may degrade the polymer and result in an increased resistivity. Further shear and/or heat exposure results from the subsequent 25 processing steps, e.g. extrusion, melt-forming, and attachment of electrodes, e.g. by l~min~tion. Conventional processing techniques provide that some of these steps, e.g.
extrusion and l~min~tion, can be performed in a continuous process, but it is common, due to the desire to ensure adequate dispersion of the filler in the polymer, to divide the m~nnf~cturing process into several discrete, i.e. separate, steps. The more times the 30 composition is heated, cooled, and subjected to shear, the greater the chances of degradation and resistivity change.

Compositions with low resistivity are desirable for use in circuit protection devices which respond to changes in ambient ten~. ldlllre and/or current conditions. Under normal 35 conditions, a circuit protection device remains in a low temperature, low resi~t~ce state in series with a load in an electrical circuit. When exposed to an ovc..;ullen~ or overtemperature condition, however, the device increases in resi~t~nre effectively ~hutting down the current CA 0226l89~ l999-0l-29 W O ~'CS~03 PCT~US97/13419 flow to the load in the circuit. For many applications it is desirable that the device have as low a resistance as possible in order to minimi7~ the effect on the resistance of the electrical circuit during normal operation. Although low resistance devices can be made by ch~nging (limen~ions, e.g. making the distance between the electrodes very small or the device area 5 very large, small devices are preferred because they occupy less space on a circuit board and generally have desirable thermal properties. The most common technique to achieve a small device is to use a composition that has a low resistivity. The resistivity of a conductive polymer composition can be decreased by adding more conductive filler, but this process can affect the processability of the composition, e.g. by increasing the viscosity. Furthermore, the 10 addition of conductive filler generally reduces the size of the PTC anomaly, i.e. the size of the increase in resistivity of the composition in response to an increase in temperature, generally over a relatively small t~ peldl~lre range. The required PTC anomaly is deterrnined by the applied voltage and the application. It is therefore necessary to minimi7~ the effects of processing which result in resistivity increases, in order to achieve a composition with 15 acceptable size and electrical plol)el~ies.

SUMMARY OF THE INVENTION

We have now found that by using a process in which a l~min~te in which a conductive 20 polymer composition is attached to a metal foil electrode (and is preferably sandwiched between two metal foil electrodes) is confl~lcte~l in a single continuous procedure, devices can be pl~ed which have low resistivity, adequate PTC anomaly, and good electrical pelrollllallce. The continuous procedure to produce the l~min~t~ allows raw, unmelted polymer and filler ingredients to be introduced into a mixing a~lus, e.g. an extruder, and 25 to be melt-formed into a l~min~te, reducing the number of steps needed to produce a device.
Unlike the conventional process in which the raw ingredients are melt-mixed and pelletized, then dried and extruded into a sheet to be l~min~te(l, the method of the invention allows the elimin~tion of the pelletizing step, along with drying of the pellets before the sheet-forming step. This means that the composition is exposed to one less heating and shearing process.
In a first aspect this invention provides a method of making a l~min~te from a conductive polymer composition which comprises (i) a polymeric component and (ii) a particulate conductive filler dispersed in the polymeric component, said method comprising (A) loading the polymeric component and the conductive filler into a mixing a~ s;

.

CA 0226189~ 1999-01-29 W 098/05503 rcTrusg7/l34l9 (B) mixing the polymeric component and the conductive filler in the mixing apparatus to form a molten mixture;

(C) transporting the molten mixture from the mixing apl)a~ s through a die;
(D) forming the molten mixture into a polymeric sheet; and (E) ~ ching metal foil to at least one side of the sheet to form a l~min~te, 10 steps (A) to (E) being conducted sequentially in a single continuous procedure.

In a second aspect, this invention provides an electrical device which ( I ) comprises (a) a resistive element which is composed of a conductive polymer composition which exhibits PTC behavior and which comprises (i) a polymeric component which has a melting temperature Tm, and (ii) dispersed in the polymeric component a particulate conductive filler; and (b) two electrodes which (i) are attached to the resistive element, (ii) comprise metal foil, and (iii) can be connPcte~l to a source of electrical power;

(2) has a resistance at 20~C, R20, of at most 50.0 ohm;

(3) has a resistivity at 20~C, P20, of at most 50.0 ohm-cm; and (4) has been made by a method which comprises (A) loading the polymeric component and the conductive filler into a mixing a~d~lls;

(B~ mixing the polymeric component and the conductive filler in the mixing ap~dldLus to form a molten mixture;

(C) transporting the molten nllx~ul~ from the mixing a~aldLus through a die;
(D) forming the molten mixture into a polymeric sheet;

CA 0226l89~ l999-0l-29 W 098/055~3 PCT~US97/13419 (E) ~tt~rhing metal foil to two sides of the sheet to form a l~min~te; and (F) cutting the l~min~te to form the device, steps (A) to (E) being conducted sequentially in a single continuous procedure.

DETAILED DESCRIPTION OF THE INVENTION

The method of the invention is used to make a l~min~te of a conductive polymer 10 composition. The conductive polymer composition comprises a polymeric component, and, dispersed in the polymeric component, a particulate conductive filler.

The polymeric component of the composition comprises one or more polymers, one of which is preferably a crystalline polymer having a crystallinity of at least 20% in its 15 unfilled state as measured by a dirr~,enlial sc~nning calorimeter. Suitable crystalline polymers include polymers of one or more olefins, particularly polyethylene such as high density polyethylene; copolymers of at least one olefin and at least one monomercopolymerisable therewith such as ethylene/acrylic acid, ethylene/ethyl acrylate, ethylene/vinyl acetate, and ethylenetbutyl acrylate copolymers; melt-shapeable 20 fluoropolymers such as polyvinylidene fluoride and ethylenettetrafluoroethylene copolymers (including terpolymers); and blends of two or more such polymers. For some applications it may be desirable to blend one crystalline polymer with another polymer, e.g. an el~ctomer or an arnorphous thermoplastic polymer, in order to achieve specific physical or thermal properties, e.g. flexibility or m~imllm exposure tc~ ucldl lre. The polymeric component has 25 a melting temperature, as measured by the peak of the endotherm of a dir~el cnllial Sc~nning calorimeter, of Tm. When there is more than one peak, as for example in a mixture of polymers, Tm is defined as the temperature of the highest telllpcldlu'e peak. The polymeric component generally comprises 40 to 90% by volume, preferably 45 to 80% by volume, especially 50 to 75% by volume of the total volume of the composition.
The particulate conductive filler which is dispersed in the polymeric component may be any suitable m~t~ri~l, including carbon black, graphite, metal, metal oxide, conductive coated glass or ceramic beads, particulate conductive polymer, or a combination of these.
The filler may be in the form of powder, beads, flakes, fibers, or any other suitable shape.
35 The ~luanlily of conductive filler needed is based on the required resistivity of the composition and the resistivity of the conductive filler itself. For many compositions the CA 0226l89~ l999-0l-29 conductive filler comprises 10 to 60% by volume, preferably 20 to 55% by volume,especially 25 to 50% by volume of the total volume of the composition.

The conductive polymer composition may comprise additional components, such as S antioxidants, inert fillers, nonconductive fillers, radiation cros~linkin~ agents (often referred to as prorads or cro.~slinking enhancers, e.g. triallyl isocyanurate), stabili~ers, dispersing agents, coupling agents, acid scavengers (e.g. CaC03), or other components. These components generally comprise at most 20% by volume of the total composition.

The composition generally exhibits positive temperature coefficient (PTC) behavior, i.e. it shows a sharp increase in resistivity with te,l,l,eldlllre over a relatively small ten~p~,.dl lre range, although the method of the invention may be used to prepare compositions which exhibit zero telllpe,dl lre coefficient (ZTC) behavior. In this application, the term "PTC" is used to mean a composition or device which has an R14 value of at least 2.5 and/or 15 an R1 oo value of at least 10, and it is p.efell.,d that the composition or device should have an R30 value of at least 6, where R1 ~ is the ratio of the resistivities at the end and the beginning of a 1 4~C range, Rl oo is the ratio of the resistivities at the end and the beginning of a 1 00~C
range, and R30 is the ratio of the resistivities at the end and the beginning of a 30~C range.
Generally the compositions used in devices of the invention which exhibit PTC behavior 20 show increases in resistivity which are much greater than those minimnm values. It is preferred that compositions used to form devices of the invention have a PTC anomaly at at least one telllp~ldlllre over the range from 20~C to (Tm + 5~C) of at least 104, preferably at least 104 5, particularly at least 105, especially at least 105 5, i.e. the log[reci~t~n.~e at (Tm +

5~C)/resistance at 20~C] is al least 4.0, preferably at least 4.5, particularly at least 5.0, 25 especially at least 5.5. If thc maximum resict~nce is achieved at a telll~ueldlule Tx that is below (Tm + 5~C), the PTC anomaly is d~ by the log(resist~n~e at Tx/resict~nce at 20~C).

The resistivity of the composition depends on the application and what type of 30 electrical device is required. When, as is preferred, the composition is used to make l~min~te for circuit protection devices, the composition has a resistivity at 20~C, P20, of at most 100 ohm-cm, preferably at most 50 ohm-cm, more preferably at most 20 ohm-cm, particularly at most 10 ohm-cm, more particularly at most 5 ohm-cm, especially at most 2.0 ohm-cm, most especially at most 1.0 ohm-cm. When the composition is used in a heater, the resistivity of 35 the conductive polymer composition is preferably higher, e.g. at least 102 ohm-cm, preferably at least 103 ohm-cm.

CA 0226l89~ l999-0l-29 W O 98/05503 PCTrUS97/13419 Suitable conductive polymer compositions are disclosed in U.S. Patent Nos.
4,237,441 (van Konynenburg et al), 4,388,607 (Toy et al), 4,534,889 (van Konynenburg et al), 4,545,926 (Fouts et al), 4,560,498 (Horsma et al), 4,591,700 (Sopory), 4,724,417 (Au et al), 4,774,024 (Deep et al), 4,935,156 (van Konynenburg et al), 5,049,850 (Evans et al), and S,250,228 (Baigrie et al), 5,378,407 (Chandler et al), 5,451,919 (Chu et al), and S,582,770 (Chu et al), and in International Publications Nos. W 096/29711(Raychem Corporation, published September 26,1996) and W 096/30443 (Raychem Corporation, published October 3,1996).
The method of the invention comprises five steps, steps (A) to (E), which are conducted sequentially in a single continuous procedure. Additional process steps, e.g. heat-tre~tmçnt or irradiation, may be conducted between two steps of the invention so long as the process remains continuous. At least parts of two steps may be conducted simultaneously, e.g. Ll~l~o~ling the molten mixture through a die (step (C)) which has a shape which forms the molten llli~lu~e into a polymeric sheet (step (D)).

In step (A), the polymeric component and the particulate conductive filler are loaded into a mixing a~p~a~us. In a preferred embodiment, both the polymeric component and the conductive filler are in the form of dry powders, flakes, fibers, or pellets which can be readily fed into the mixing appa~dl~ls. Although these two components can be fed separately into the mixing ~pard~us, preferably the polymeric component and the conductive filler are "premixed" in the dry state, e.g. by means of a blender or mixer such as a HenschelTM mixer, to improve the uniformity and flow of the components during the loading step. Additional components, in the form of powder, pellets, or liquid, may be premixed with the polymer component and the particulate component, or may be added at different points in the process.
Loading may be achieved by any means, although loss-in-weight feeders such as those sold by K-Tron America under the tr~(lçn~rne "K-Tron", are preferred to ensure consistent feeding into the al)palaLus. The mixing al,p~ s is preferably an extruder, although other types of 30 mixing e4~;p~ including internal mixers such as BanburyTM mixers, BrabenderTM mixers, and MoriyamaTM mixers, may be used with suitable at~ ments for conveying material to complete the required steps of the invention. Suitable extruders include single screw extruders, co-rotating twin screw extruders, counter-rotating twin screw extruders, or reciprocating single screw extruders, e.g. a BussTM kn~der. When an extruder is used, various additives, e.g. cro~linking agents such as peroxide, can be added continually at a feed port do~,~ eanl from the port in which the polymeric component and conductive filler are introduced. Unlike conventional methods in which adding a cros.slinking agent to a .

CA 0226l89~ l999-0l-29 WO ~8/0'50~ PCT~US97/13419 composition would result in a crosslinked mass which could not easily be subsequently formed into a uniform sheet or other shape, the method of the invention is particularly suited for in-line chemical cros~linkin~. The continuous procedure allows the cros~linking agent to be added just before the material is transported through a die.
In step (B), the polymeric component and the conductive filler are mixed in the mixing app~lus to form a molten mixture, i.e. one which has a te,llpeldl~re above the melting temperature Tm of the polymeric component. During step (B), the conductive filler, as well as other components such as inorganic fillers or pigments, is dispersed in the 10 polymeric component. To ensure that adequate mixing and dispersion is achieved, the screw of an extruder may be ~çsign~cl to have mixing or kn~-ling sections, as well as conveying sections. For example, we have found that incorporating kne~ling sections in at least 10% of the total screw length for a corotating twin screw extruder has produced acceptable dispersion. When an extruder is used, it is ple~ ,d that the ratio of the screw length to its diameter, i.e. the L/D ratio, is at least 10:1, preferably at least 15:1, particularly at least 20:1, especially at least 30:1, e.g. 40:1, in order to achieve adequate dispersion of the conductive filler. The mixing al~p~dllls may be heated, e.g. electrically or by oil, in one or more sections (zones). A vacuum app~udlus, to remove volatiles generated during mixing, can be positioned appropriately in combination with the mixing appa~dlus.
In step (C), the molten mixture is transported from the mixing appaldlus through a die. The term "die" is used in this specification to mean any element which has an orifice through which the molten material can pass. Thus a die may be a mold, a nozzle, or an article with an opening or gap of a particular shape through which the molten material passes. The 25 die can be att~checl directly to an exit port of the mixing ap~ s, e.g. by means of an adapter, or it may be separated from the mixing appa~dlus by one or more pieces of equipm~nt, e.g. a gear pump or a vacuum app~dl~ls. When the mixing apl,~dl~ls is an extruder, the "transporting" of the molten mixture occurs during the normal operation of the extruder. Other means of transporting the molten mixture may be required if other types of 30 mixing apparatus are used.

In step (D), the molten mixture is formed into a polymeric sheet. This can be achieved easily by extrusion through a sheet die or by calendering the molten mixture, i.e.
passing the molten mixture between rollers or plates to thin it into a sheet. The thickness of 35 the calendered sheet is detçrminecl by the ~ t~nre between the plates or rollers, as well as the rate at which the rollers are rotating. Generally the polymeric sheet has a thickness of 0.025 to 3.8 mm (0.001 to 0.150 inch), preferably 0.051 to 2.5 mm (0.002 to 0.100 inch). The CA 0226189~ 1999-01-29 W O 98/05503 PCTrUS97113419 polymeric sheet may have any width. The width is determined by the shape and width of the die or the volume of material and rate of calendering, and is often 0.15 to 0.31 m (6 to 12 inches).

In step (E), a l~min~te is formed by ~tt~rhing metal foil to at least one side, preferably to both sides, of the polymeric sheet. When the l~min~te is cut into an electrical device, the metal foil layer(s) act(s) as an electrode. The metal foil generally has a thickness of at most 0.13 mm (0.005 inch), preferably at most 0.076 mm (0.003 inch), particularly at most 0.051 mm (0.002 inch), e.g. 0.025 mm (0.001 inch). The width ofthe metal foil is generally approximately the same as that of the polymeric sheet, but for some applications, it may be desirable to apply the metal foil in the form of two or more narrow ribbons, each having a width much less than that of the polymeric sheet. Suitable metal foils include nickel, copper, brass, aluminum, molybdenllm, and alloys, or foils which comprise two or more of these materials in the same or different layers. Particularly suitable metal foils have at least one surface that is electrodeposited, preferably electrodeposited nickel or copper. Appropriate metal foils are disclosed in U.S. Patents Nos. 4,689,475 (Matthiesen), and 4,800,253 (Kleiner et al), and in Tnt~rn~tional Publication No. W 095/34081 (Raychem Corporation, published December 14, 1995). In a preferred embodiment, the metal foil contacts the polymeric sheet and is then passed through rollers, e.g. via a roll stack, to promote good l~min~tion of the foil to the polymer. In addition, to minimi7P cooling of the sheet as it exits the die, it is preferred that the distance between the die and the roll stack be relatively small, e.g. Iess than 0.61 m (2 feet), preferably less than 0.31 m (1 foot). For some applications, an adhesive composition (i.e. a tie layer) may be applied to the polymeric sheet, e.g. by spraying or brushing, before contact with the metal foil. The l~min~te resulting from step (E) may be wound onto a reel or sliced into discrete pieces for further procç~ing or storage. The thickness of the l~min~te is generally 0.076 to 4.1 mm (0.003 to 0.160 inch).

The method of the invention can be used to produce a l~min~te with more than onepolymeric sheet by using two or more mixing app~dlus/transporting/forming set-ups which produce polymeric sheets based on the same or different polymeric components andconductive fillers.

When the l~min~te comprises two metal foils, it can be used to form an electrical device, particularly a circuit protection device. The device may be cut from the l~min~te in step (F). In this application, the term "cut" is used to include any method of isolating or St;~dld~ g the device from the l~min~te e.g. dicing, punching, shearing, cutting, etching and/or breaking as described in lnt~rn~tional Publication No. WO 95/34084 (Raychem CA 0226189~ 1999-01-29 Corporation, published December 4, 1995), or any other suitable means. Step (F) may, but need not, be part of the single continuous procedure of steps (A) to (E). Additional metal leads, e.g. in the form of wires or straps, can be attached to the foil electrodes to allow electrical connection to a circuit. In addition, elements to control the thermal output of the ~ S device, e.g. one or more conductive terrnin~l.c, can be used. These tçrrnin~ls can be in the forrn of metal plates, e.g. steel, copper, or brass, or fins, that are ~tt~rlle~l either directly or by means of an intermçtli~te layer such as solder or a conductive adhesive, to the electrodes.
See, for example, U.S. Patent Nos. 5,089,801 (Chan et al) and 5,436,609 (Chan et al). For some applications, it is pl~rell~d to attach the devices directly to a circuit board. Exarnples of such attachment techniques are shown in Tntern~tjonal Publications Nos. WO94/01876 (Raychem Corporation, published January 20, 1994) and WO95/31816 (Raychem Corporation, published November 23, 1995).

In order to improve the electrical stability of the device, it is often desirable to subject the device to various proce~sin~ techniques, e.g. crocclinking and/or heat-tre;ltment Crosslinkin~ can be accomplished by chemical means or by irradiation, e.g. using an electron bearn or a Co60 r irradiation source. The level of crosslinking depends on the required application for the composition, but is generally less than the equivalent of 200 Mrads, and is preferably subst~nti~lly less, i.e. from 1 to 20 Mrads, preferably from I to 15 Mrads, particularly from 2 to 10 Mrads for low voltage (i.e. less than 60 volts) circuit protection applications. Generally devices are cros~linke~l to the equivalent of at least 2 Mrads. Various proce~ing procedures for devices are described in International Publication No.
W096/29711 (published September 26, 1996).

Devices of the invention are preferably circuit protection devices that generally have a re~ist~nce at 20~C, R20, of less than 100 ohms, preferably less than 50 ohms, particularly less than 20 ohms, more particularly less than 10 ohms, especially less than 5 ohms~ most especially less than 1 ohm. Because l~min~te prepared by the method of the invention comprises a conductive polymer composition which can have a low resistivity, it can be used to produce devices with very low resistances, e.g. 0.001 to 0.100 ohm. Devices which are heaters generally have a resistance of at least 100 ohms, preferably at least 250 ohms, particularly at least 500 ohms It is to be understood that the l~rnin~te made by the method of this invention can be used for any type of electrical device, e.g. heaters or sensors, as well as circuit protection devlces.

CA 0226189~ 1999-01-29 W O9X,'1550~ PCTrUS97/13419 The invention is illustrated by the following Examples in which Examples 1, 2, 4, 6, 8, and 10 are Co~ e Examples.

Examples 1 to 7 For each Example, the following ingredients, in the weight percentages based on the weight of the total composition listed in Table I, were mixed at 1500 rpm for three minutes using a HenschelTM mixer: PVDF (KFTM l OOOW, polyvinylidene fluoride in powder form with a melting tempelalul~ of about 177~C, available from Kureha), ETFE (TefzelTM HT
2163, ethylene/ tetrafluoroethylene/perfluorinated butyl ethylene terpolymer with a melting t~ cldLllre of about 235~C, available from DuPont), CB (RavenTM 430, carbon black, available from Columbian Chemicals), TAIC (triallyl isocyanurate), and CaCO3 (AtomiteTM
powder, calcium carbonate, available from John K. Bice Co.) The mixed dry ingredients were then subjected to either a two-step process (conl~dlive) or a one-step process of the invention.

Two-Step Process For Conlp~ati\~e Examples 1, 2, 4, and 6, the mixed dry ingredients were introduced into a corotating twin screw extruder using a screw with an L/D ratio of 40: 1 (ZSK-40, available from Werner-Pfleiderer), mixed, extruded into strands, and cut into pellets.

For Example 1, the pellets were dried at 80~C (175~F) for at least 24 hours and then were extruded through a 25 mm (1 inch) single screw extruder fitted with a nozzle with a diameter of 9.5 mm (0.375 inch). The molten m~teri~l was extruded from the nozzle and fed onto a roll stack positioned about 25 mrn (1 inch) from the nozzle end. The roll stack was used to both calender the m~teri~l into a sheet with a thickness of about 0.250 mm (0.010 inch) and a width of about 114 to 152 mm (4.5 to 6.0 inches) and to attach electrodeposited nickel/copper foil (Type 31, 1 -oz foil having a thickness of about 0.044 mm (0.0017 inch) available from Fukuda) to both sides of the c~lenclered sheet. The resulting l~ nin~te had a thickness of about 0.34 mm (0.0135 inch).

For Examples 2, 4, and 6, the pellets were extruded through a counter-rotating/co-rotating twin-screw extruder (ZSE-27, available from Leistritz) in a corotation mode using a screw with no kn~rling elements and having an L/D ratio of 40: 1. The extruder was fitted with a gear purnp having a capacity of 10 cm3/revolution (Pep II, available from Zenith) and CA 0226189~ 1999-01-29 W 098/05503 PCTrUS97113419 then with a nozzle as above. The material was extruded, c~lenrlered, and l~min~ted following the same procedure as for Example 1.

One-Step Process The mixed dry ingredients were introduced into a ZSE-27 extruder used in a corotation mode, and having a screw configuration in which 1 1% of the total screw length was kne~-ling elements. The screw had an L:D ratio of 40:1. The extruder was fitted with a gear pump and nozzle as in Examples 2, 4, and 6. The material was mixed, and the mixed 10 m~teri~l was extruded through the gear pump dnd nozzle, calendered, and l~min~ted following the same procedure as for Examples 2, 4, and 6.

Device Plepdldlion For Examples 2 to 7, the l~min~te was irradiated in a continuous process using a 3.5 MeV electron beam to a total of 7.5 Mrad. The l~min~te was then coated in a continuous process with solder (using a solder telllpel~luf~ of about 250~C), and devices with ~1imen~ions of 11 x 15 mm (0.43 x 0.59 inch) were punched from the l~min~te. Two 20 AWG tin-coated copper leads about 25 rnm (1 inch) long were ~tt~ched to the device and the device was t~ e.dl~lre cycled at a rate of 10~C/minute from 40~C to 160~C to 40~C for six cycles, with a 30 minute dwell time at the temp~,ldlule extremes for each cycle.

For Example 1, devices with ~limen~ions as above were cut from l~min~te that hadbeen irradiated as discrete pieces and had not been solder-coated. Leads were att~eh~d and the devices were temperature cycled as above.

CA 0226189~ 1999-01-29 W O 98/05503 PCTrUS97/13419 TABLE I

Example ¦ 1 * ¦2* ¦ 3 ¦ 4* ¦ 5 ¦ 6* ¦ 7 Composition in weight% (density in g/cm3) PVDF (1.76) 52.90 52.90 52.90 58.20 58.20 56.43 56.43 ETFE (1.70) 5.89 5.89 5.89 6.47 6.47 6.27 6.27 CB (1.8) 37.23 37.23 37.23 31.35 31.35 33.32 33.32 TAIC (1.158) 2.00 2.00 2.00 2.00 2.00 2.00 2.00 CaCO3 (2.71) 1.98 1.98 1.98 1.98 1.98 1.98 1.98 Process Steps 2 2 1 2 1 2 *Col,lp~dlive Exarnples Device Testin~

The device resi~t~nce at 20~C was measured and the resistivity calculated. The resistance versus temperature prol.c, lies of the device were determin~d by positioning the device in an oven and measuring the resistance at intervals over the temperature range 20 to 200 to 20~C. The height of the PTC anomaly, PTC, was determined after the first tcl"~e~al~e cycle as log(resistance at 175~C/resistance at 20~C).

The results, shown in Table II, indicate that the devices made by the method of the invention had lower initial resistances than devices made by the conventional process. In addition, the method of the invention produced devices which had PTC anomalies similar to those made by the convcntional method even though the composition for the devices of the invention contained less conductive filler (compare Example 5 with Example 6 and Example 7 with Example 2).

TABLE II

Example 1 * 2* 3 4* 5 6* 7 Ro (mohm) 17.9 14.6 10.9 38.3 23.7 26.1 15.6 P20 (ohm-cm) 1.11 0.95 0.71 2.49 1.54 1.69 1.01 PTC (~lec~des) 4.1 4.4 3.9 5.5 5.5 5.3 4.5 *Co"~p~alive Examples CA 0226189~ 1999-01-29 W O 98/OSS03 PCT~US97/13419 Examples 8 to 11 The following ingredients, in the weight percentages listed in Table III, were mixed in a Henschel mixer: HDPE (PetrotheneTM LB832, high density polyethylene having a melting temperature of about 135~C, available from Quantum Chemical), EBA (Fn~theneTM 705 009, ethylene/n-butyl acrylate copolymer having a melting temperature of about 105~C, available from Quantum Chemical), and CB (Raven 430). The mixed dry ingredients were then subjected to either a two-step process or a one-step process.

10 Two-Step Process For Colllpaldlive Examples 8 and 10, the mixed dry ingredients were introduced into a 70 mm (2.75 inch) Buss kn~der (a reciprocating single screw extruder), mixed, extruded into strands, and cut into pellets. A ZSE-27 extruder in a corotation mode with a screw having no 15 kn~-ling elements and an L:D ratio of 40: I was fitted with a gear pump at the exit port of the extruder as in Example 2. The gear pump was attached to a sheet die having an opening 152 mm (6 inch) wide and 0.038 mm (0.0015 inch) thick. The pellets were extruded through the sheet die to form a polymeric sheet and the polymeric sheet was drawn from the die onto a roll stack spaced about 12.7 mm (0.5 inch) from the die lip and having rubber-coated rollers 20 heated to about 155~C. Nickel/copper foil as described in Example 1 was l~min~t~cl onto the polymeric sheet. The resulting l~min~te had a thickness of about 0.127 mm (0.005 inch).

One Step Process For Examples 9 and 11, the mixed dry ingredients were introduced into a Z~E-27 extruder used in a corotation mode, and having a screw configuration in which 11 % of the total screw length was kne~tling elements and the L:D ratio was 40:1. The material was mixed in the extruder, and continuously extruded and l~rnin~t~d using the gear pump, die, and l~min~tion process described for Example 8.
Device Pl~dld~ion T .~min~te was solder coated in a continuous (using a solder te.np~lalule of about 220~C), and devices with dimensions of 5 x 12 mm (0.20 x 0.47 inch) were punched from the 35 l~min~te. The devices were heat-treated in a process that exposed them to a tt;nl~ d~ule of 185 to 215~C for about 4 seconds. The devices were then cros~link~d to 10 Mrad using a Co60 gamma irradiation source. Nickel leads with dimensions of 0.13 x 5 x 13.5 mm (0.005 x CA 0226189~ 1999-01-29 wogX/!~re~3 PCTrUS97/13419 0.2 x 0.5 inch) were ~ he~l to the electrodes on both sides of the device by solder reflow and the devices were temperature cycled six cycles from -40~C to 85~C with a 30 minute dwell time at the t~lnpr~ldlule extremes.

5 Device Testin~

Devices were tested as above except that the PTC anomaly was measured over a tclnp~ldlule range of 20 to 160 to 20~C. The height of the PTC anomaly was determined at three different temperatures, 105~C, 125~C, and 140~C as log (resistance at Ty~C/resistance at 10 20~C) for the second cycle, where y was the measurement tellll)c~dLure. (The measurement at 140~C was closest to the actual melting pointing of the conductive polymer composition.) The results, shown in Table III, indicated that similar PTC anomalies could be achieved, but with a much lower resi~t~nre, for a composition made with the one-step continuous process rather than a conventional two-step process (compare Examples 8 and 9).
TABLE III

F~mple ¦ 8* ¦ 9 ¦ 10* ¦ 11 Composition in weight % (density in ~/cm3) HDPE (0.954) 5.0 5.0 4.8 4.8 EBA(0.922) 45.0 45.0 43 0 43 0 CB (1.8) 50.0 50.0 52.2 52.2 Ro (mohm) 44.6 33.4 28.6 22.8 PTCIos 5.42 4.73 4.35 3.76 PTC12s 8.95 8.12 7.50 6.62 PTCI40 9.35 8.28 7.64 6.73 *Coll~p,Jl~Li~e Examples

Claims (10)

What is claimed is:

1. A method of making a laminate from a conductive polymer composition which comprises (i) a polymeric component and (ii) a particulate conductive filler dispersed in the polymeric component, said method comprising (A) loading the polymeric component and the conductive filler into a mixing apparatus;

(B) mixing the polymeric component and the conductive filler in the mixing apparatus to form a molten mixture;

(C) transporting the molten mixture from the mixing apparatus through a die;

(D) forming the molten mixture into a polymeric sheet; and (E) attaching metal foil to at least one side of the sheet to form a laminate, steps (A) to (E) being conducted sequentially in a single continuous procedure.

2. A method according to claim 1 wherein the mixing apparatus is an extruder, preferably a single screw extruder, a co-rotating twin screw extruder, a counter-rotating twin screw extruder, or a reciprocating single screw extruder.

3. A method according to claim 1 wherein the die is a nozzle.

4. A method according to claim 1 wherein the die is directly attached to an exit port of the mixing apparatus.

5. A method according to claim 1 wherein step (E) comprises attaching metal foil to two sides of the sheet.

6. A method according to claim 1 wherein the conductive polymer composition in the laminate has a resistivity of less than 20 ohm-cm, preferably less than 10 ohm-cm.

7. A method according to claim 1 wherein the polymeric sheet has a thickness of 0.025 to 3.8 mm (0.001 to 0.150 inch), and is formed by extrusion or calendering.

8. An electrical device which (1) comprises (a) a resistive element which is composed of a conductive polymer composition which exhibits PTC behavior and which comprises (i) a polymeric component which has a melting temperature T m, and (ii) dispersed in the polymeric component a particulate conductive filler; and (b) two electrodes which (i) are attached to the resistive element, (ii) comprise metal foils, and (iii) can be connected to a source of electrical power;

(2) has a resistance at 20°C, R20, of at most 50.0 ohm;

(3) has a resistivity at 20°C, P20, of at most 50.0 ohm-cm; and (4) has been made by a method according to claim 5 which further comprises (F) cutting the laminate to form the device.

9. A device according to claim 8 wherein the polymeric component comprises a crystalline polymer which is a polyethylene, an ethylene copolymer, a fluoropolymer, or a mixture of these polymers, and the particulate filler comprises carbon black.

10. A device according to claim 8 which has a PTC anomaly of at least 10 4Ø